The present invention relates to a device for the extinguishment of fires by injecting foam and/or dry chemical powder and a fixed/floating roof storage tank incorporating the device.
The device of the present invention pertains to the oil and petrochemical industries from the viewpoint of fire protection of fixed/floating roof flammable liquid storage tanks.
The present state-of-the-art of extinguishment of oil fires in fixed/floating roof storage tanks implies use of foam and/or dry chemical powder.
In case of large scale fires in flammable liquid storage tanks, foam is practically used as an efficient fire extinguishing agent. Foam is an aggregate of air-filled bubbles formed from aqueous solutions and is lower in density than the lightest flammable liquids. Foam is known for its fire knock-down capability and for blanketing the flammable liquid surface on which it is applied. It is principally used to form a coherent floating blanket on flammable and combustible liquids lighter than water and prevents or extinguishes fire by excluding air and cooling the fuel. It also prevents reignition by suppressing formation of flammable vapors. It has the property of adhering to surfaces, providing a degree of exposure protection from adjacent fires. Foam may be used as a fire prevention, control, or extinguishing agent for flammable liquid storage tanks or processing areas. At present foam solution for these hazards is supplied by fixed piping arrangement either by top surface application or by subsurface/semi-subsurface application. The major drawback of the present top surface application methods is that these methods get damaged and thus render practically to be inefficient/ineffective when the roof of the tank gets blown off or otherwise when tank shell at the roof level gets deformed and/or damaged/buckled due to severe heat exposure during fire; and that of subsurface/semi-subsurface application is that the foam bubbles of the foam being injected carry along with them a part of the flammable liquid being embedded onto the bubble surface, resulting ultimately in the disintegration of foam bubbles at the top surface of the burning flammable liquid; thus jeopardizing the purpose for which it has been used.
Dry chemical powder extinguishant is used mainly for knocking down the flammable liquid fires. Dry chemical powder is a fine amorphous mixture which is used as a fire extinguishing agent. The principal base chemicals used in the production of currently available dry chemical extinguishing agents are sodium bicarbonate, potassium bicarbonate, potassium chloride, urea potassium bicarbonate, and monoammonium phosphate. Various additives are mixed with these base materials to improve their storage, flow, and water repellency characteristics. The most commonly used additives are metallic stearates, tricalcium phosphate, or silicones, which coat the particles of dry chemical powder to make them free flowing and resistant to the caking effects of moisture and vibration. Borax and sodium bicarbonate based dry chemical powder were the first such agents developed. Sodium bicarbonate became the standard agent because of its greater effectiveness as a fire extinguishing agent. About 1960, sodium bicarbonate based dry chemical powder was modified to render it compatible with protein based low expansion foams to permit a dual agent attack. Presently, there are five basic varieties of dry chemical extinguishing agents. Particles of dry chemical fire extinguishing agents range in size from less than 10 microns up to 75 microns. Dry chemical powder is stable at both low and normal temperatures. However, since some of the additives may melt and cause sticking at higher temperatures, an upper storage temperature limit of 49.degree. C. is recommended for dry chemical powder, which in special cases may be acceptable up to 66.degree. C. for very short durations. At fire temperature, the active ingredients either dissociate or decompose while performing their function in fire extinguishment. Of extreme importance is the danger caused by indiscriminate mixing of the various dry chemical powders. Fire tests on flammable liquids have shown potassium bicarbonate based dry chemical powders in extinguishment. Similarly, monoammonium phosphate has been found to be equal to or better than sodium bicarbonate in extinguishment effectiveness (Guise 1962). The effectiveness of potassium chloride is about equivalent to potassium bicarbonate, and urea-potassium bicarbonate exhibits the greatest effectiveness of all the dry chemical powders tested. At present, the dry chemical powder is intended for application by means of portable extinguishers, hand hose line system, or fixed systems. In all these cases, the dry chemical powder is introduced by top surface application methods. In fixed systems, they are fed by fixed piping arrangements. When applied directly to the fire area, dry chemical powder causes the flame to go out almost at once. Smothering, cooling, and radiation shielding contributes to the extinguishing efficiency of the chemical powders, but studies suggest that a chain-breaking reaction in the flame zone is the principal cause of extinguishment (Haessler 1974).
The drawbacks associated with the presently available devices are: combination of foam and dry chemical powder for extinguishment of flammable liquid fires in storage tanks is presently being used, either by top surface application or by subsurface/semi-subsurface applications. The drawback of top surface application is that when the roof of the fixed/floating roof tank gets blown off, the top surface application device(s) gets damaged and is unable to perform its function. In case of subsurface/semi-subsurface application, the foam bubbles carry along with them the flammable liquid and thus add to the fire and moreover the foam bubbles get broken off due to immediate heat contact and increase in the surface tension of the bubble surface because of the embedded flammable liquid; thus fueling the fire and, ultimately, rendering themselves ineffective in extinguishing the fire; whereas in case of the dry chemical powder, the major drawback is that it cannot be applied by using subsurface/semi subsurface application methods.